You've got the right idea with air, fuel, and spark.
Air is almost never the culprit, in my experience. Pretty hard to starve the engine of air to the point where it won't even start.
Spark could be the culprit. Could be caused by a multitude of things. But to see if you are getting spark, you could buy a cheap spark plug tester like this:
Fuel has usually been the culprit with my experience with these vans. On one van I had the fuel pump seize up. When everything is working right, you should turn the key to the "on" position (but not past, which would cause the starter motor to start cranking the engine). When the key first moves to the "on" position, you should hear the fuel pump turn on (a whirring sound for a few seconds) which primes the fuel system. One day my van wouldn't start, and after some troubleshooting I realized I wasn't hearing the fuel pump turn on. Turns out my fuel pump was seized, and whacking the fuel tank with a rubber mallet was enough to break it free. You would want to replace that pump eventually.
I've also had the PCM go bad on two old fords (one econoline and one F150). There are plenty of services that offer refurbished PCMs at a reasonable cost. The symptom was very similar to the above - the fuel pump would not turn on when the key was switched to the "on" position.
Marret's suggestion above for using some starting fluid is good as well. If it starts running with the starting fluid, but then dies out, that tells you that the van CAN run with some fuel, but isn't getting the fuel it needs.
Also as Marret mentioned, check the battery. Pull it out and take it to an auto parts store. They will all test a battery for free.
Obviously this is not a totally inclusive list, but it should be a good starting place. Generally when troubleshooting things like this it helps to think logically step by step, instead of blindly replacing parts (e.g. someone on the internet says "my PCM went bad, so go replace that first"). Test one variable at a time to narrow down what the cause of the issue is.